Despite the conspicuous and environmentally corrosive nature of urbanization, tracking it has always been troublesome. Urban sprawl moves relatively fast, consuming hundreds of square miles of land in North America alone each year, and it tends to spread over the landscape in an irregular, organic fashion like mold on a piece of fruit. Even when viewed in conventional satellite imagery, urbanization’s outer reaches often appear blurry and indistinguishable from surrounding forest and farmland. As such, scientists have traditionally had a hard time accurately mapping and thus understanding the large-scale effects of our ever-growing cities and suburbs on the ecosystem and biodiversity.

Not too long ago, Marc Imhoff, a biologist at National Aeronautics and Space Administration’s (NASA) God-dard Space Flight Center, came across a solution to this modern dilemma. He developed a technique for outlining urbanization over entire continents by using satellite images of the illumination given off by cities at night. The maps provided an unsettling picture as to how urbanization may be threatening biodiversity across the whole of Canada and the U.S.

Seeing the Light

After several years of searching, Imhoff found his method for tracking urban sprawl in probably the least obvious place. At an amateur astronomy meeting, Imhoff and his fellow enthusiasts were discussing the growing problem of light pollution. One of them non-chalantly pulled out a set of satellite images displaying the illumination cities and towns in the U.S. generate at night. As the rest of the club lamented over how the city lights limit their view of the heavens, Imhoff realized he had his map of human habitat.

The images were taken by a U.S. Air Force satellite network originally designed to aid aircraft navigation by detecting the lunar illumination off of nighttime clouds. What the Air Force uncovered is that during a new moon, the satellite was sensitive enough to record the lights emanating from cities. Over a period of several new moons, the satellite data could be pieced together to produce a global image of city lights.

The light maps the Air Force provided were initially too bright and misrepresented the size of urban areas by a factor of eight. By first toning down the city lights data and then combining them with statistics from the U.S. Census Bureau, Imhoff managed to construct more accurate maps displaying urban areas of the U.S. by population density. All land area in the country was mapped into three categories: congested urban areas with greater than 1,000 people or more per 2.5 square kilometers, strip-mall laden peri-urban areas with around a hundred people or more per 2.5 square kilometers, and rural areas with less than 100 people. He fashioned subsequent maps for Canada, China, and Egypt.

Imhoff first used the maps to study trends in land management, which yielded a few interesting, if not disturbing, results. In one such study, he effectively laid the urbanization maps on top of soil quality maps to show that people worldwide have the habit of building on and thus permanently degrading some of the most arable land. A separate investigation revealed that urbanization changes both the density and growing season of vegetation. Urban sprawl in Chicago, for instance, diminishes the density of the indigenous mixed forest, and heat from the city elongates the growing season. The result is a net loss of yearly plant growth. But in Denver, both density and growing season increase, resulting in a net gain over the surrounding dry scrubland.

Homing In

More recently, Imhoff teamed up with fellow Stanford graduate Taylor Ricketts, who is now the Director of the Conservation Science Program at World Wildlife Fund (WWF), to observe the relation between urbanization and biodiversity. Imhoff contacted Rickets in 1999 after reading an article on Ricketts’s work in a newsletter sent out by the Center for Conservation Biology at Stanford.

Ricketts, a graduate student at the time, had just spent three years leading one of the most extensive biodiversity studies ever done for Canada and the U.S. The WWF/Stanford team first combined a patchwork of previously rendered ecological maps of Canada and the U.S. into one large map with 116 distinct, color-coded, numbered ecoregions. From Florida’s soggy swamps to Alaska’s frozen tundra, each region on the map represented an area with a unique mix of plants, animals, and environmental conditions. They then chased down the available range data for thousands of plant and animal species in North America. They gleaned some of the range data, especially for birds, from everyday hobbyist publications such as Peterson’s Field Guides. Other sources of data, such as that for tiger beetles, they found buried in computer data in the backrooms of musty, university biology departments.

By matching maps of the species ranges to the ecoregions map, they determined the number of species exclusive, or endemic, to each region and the richness of species in each region.

Upon reviewing Ricketts’s work, Imhoff reasoned that they could get a good measure of where human habitat most threatened natural habitats by combining this biodiversity data with his urbanization data and the U.S. Geological Survey agricultural map. In a study published this past October in Conservation Ecology (1), the scientists compared both species richness and endemism to the amount of urban area and farmland present for 110 of the regions on Ricketts’s map. They then zeroed in on those regions where high species richness and endemism seemed to be colliding most with high levels of farmland and urban area.

Shared Attractions

Right off, Ricketts and Imhoff’s study revealed a disturbing trend. Urban sprawl appears to be creeping into regions with the highest number of species all across Canada and the U.S.. Although endemism hardly coincided at all with agriculture or urbanization, high levels of urbanization and to a lesser extent farmland typically were found in the areas with high species richness.

In short, people are drawn to the same things as most plants and animals—arable soil, a temperate climate, and abundant water. We also like to be near the coasts, where habitats and species vary the most. Driving through Kansas, for instance, the change in scenery, vegetation, and animals is minimal at best. A drive in from the California coast in light traffic can quickly yield coastal, mountain, and then high desert habitat.

As to what this means for conservation efforts, Ricketts feels that we must do more than protect wildlife and create reserves in the middle of Montana or Arizona where very few people live. Previous studies have shown that urbanization reduces biodiversity in just about every ecosystem. As our cities inevitably continue to grow and expand, we should preserve the natural habitat in and around urban areas as well. Whether this means zoning for high density living or placing nature reserves and corridors amidst aluminum-sided colonials and bland restaurant chains, Ricketts admits, is still a topic for hot debate.

Currently, Imhoff and Ricketts are working on two additional projects that build on this effort. For the first, they are combining the urban and agricultural maps with data on the body size, range size, and other characteristics relevant to endangered species to see if they can develop a list of factors that predict species endangerment. As to the other study, the researchers are analyzing some of the economic costs involved in setting up wildlife reserves in and around urban areas. With any luck, such work will provide future conservationists and city planners with the information needed to take urban expansion in a wise and less destructive direction.

Imhoff, M.L. et al. 2000. The use of multisource satellite and geospatial data to study the effect or urbanization on primary productivity in the United States. IEEE Transactions Geoscience and Remote Sensing 38(6):2549-2556.

Imhoff, M.L. et al. 2004. The consequences of urban land transformation for net primary productivity in the United States. Remote Sensing of Environment 89:434-443.